1. Field of the Invention
The present invention relates to electrolytic manganese dioxide to be used as a cathode active material in e.g. manganese dry cells, particularly alkali-manganese dry cells, and a method for its production and its application.
2. Discussion of Background Information
Manganese dioxide is known as a cathode active material for e.g. manganese dry cells or alkali-manganese dry cells and has advantages such that it is excellent in storage stability and inexpensive. In particular, the alkali-manganese dry cells employing manganese dioxide as a cathode active material, are excellent in the discharge characteristics under heavy loading. Thus, they are widely used in applications ranging from digital cameras, portable tape recorders and portable information apparatus to game machines and toys, and there is a rapidly growing demand for them.
However, alkali-manganese dry cells have had a problem such that the utilization rate of manganese dioxide as a cathode active material decreases with the increase of the discharge current and manganese dioxide can't use in such low discharge voltage condition whereby the discharge capacity will be substantially impaired. Namely, the alkali-manganese dry cells have had a drawback that when they are used in apparatus using a large current (for high rate discharge), manganese dioxide packed as a cathode active material is not sufficiently utilized, and the useful time is short.
Therefore, excellent manganese dioxide capable of providing a high capacity and long useful life even under such a high rate pulse discharge condition that a large current is taken out in a short period of time, i.e. manganese dioxide excellent in a so-called high rate discharge characteristic, is desired.
In applications where the high rate discharge characteristic is required, electrolytic manganese dioxide having a high potential (hereinafter referred to as alkali potential) as measured in a 40% KOH aqueous solution by using a mercury/mercury oxide reference electrode as a standard, has been used as a cathode active material in order to increase the voltage at the time of the discharge of the cell. However, the alkali potential of conventional electrolytic manganese dioxide has not yet been sufficiently high.
Further, as electrolytic manganese dioxide having a high alkali potential, electrolytic manganese dioxide obtained by controlling the electrolytic conditions, e.g. electrolytic manganese dioxide produced by using the electrolyte containing the high acid concentration of sulfuric acid, has been proposed (Non-patent Document 1, Patent Document 1). However, in electrolysis under a production condition where the acid concentration in the electrolyte is high, electrolytic manganese dioxide electrodeposited during the electrolysis will exfoliate from the electrode for electrolysis, whereby electrolytic manganese dioxide can not constantly be produced, and the obtainable electrolytic manganese dioxide tends to have a small crystallite diameter and a large BET surface area, whereby the packing efficiency in a cell can not sufficiently be improved, and there has been a problem that the volume energy density is low.
On the other hand, a method for producing electrolytic manganese dioxide having a high alkali potential by electrolysis at a low current density has been reported (Patent Document 2). However, the electrolytic manganese dioxide by the electrolysis at a low current density has had a problem such that the electrodeposition speed is slow whereby the productivity is low, and the crystallite diameter of the electrolytic manganese dioxide tends to be too large, whereby the reactivity of the electrolytic manganese dioxide tends to be poor, and the discharge capacity as a cathode active material for cells tends to decrease.
Further, a method for producing manganese dioxide having a high alkali potential has been proposed wherein hydrochloric acid is used instead of sulfuric acid which is commonly used in the electrolyte (Patent Document 1). However, in the electrolysis using hydrochloric acid, there will be many inconveniences from the viewpoint of the production such that chlorine is generated during the electrolysis, and a further countermeasure is required, and there have been problems that the electrolytic manganese dioxide thereby obtainable has a small crystallite diameter, whereby the packing property in a cell can not be improved, and the volume energy density is low.
Further, the electric discharge of electrolytic manganese dioxide in alkali-manganese cells is known to proceed by the formula (1) to obtain a proton (H+) from water (H2O).MnO2+H2O+e−→MnOOH+OH−  (1)
Particularly, in the high rate discharge, quick dispersion of H+ in electrolytic manganese dioxide is necessary, and for that purpose, it is effective that it has many H+ in its structure. H+ in the electrolytic manganese dioxide is present in the structure in the form of a hydroxyl group (—OH) or crystal water (H2O), and among such a hydroxyl group or water of crystallization (excluding adhesive moisture), water (H2O) to be desorbed under a certain heating condition is referred to as structural water.
Heretofore, as a method to obtain electrolytic manganese dioxide having a high alkali potential, a method of treating electrolytic manganese dioxide with a sulfuric acid solution, has been reported (Patent Document 3). However, the manganese dioxide obtained by such conventional sulfuric acid treatment did not have a sufficiently high alkali potential.
Further, as electrolytic manganese dioxide excellent in the high rate discharge characteristic, electrolytic manganese dioxide having a potential of at least 250 mV and having a composition wherein the molar ratio of H to Mn in the structure is at least 0.30 and at most 0.40, is is known wherein the molar ratio of structural water to manganese in the electrolytic manganese dioxide is at least 0.15 and at most 0.20 (Patent Document 4). However, the alkali potential of electrolytic manganese dioxide obtained by such a method was at most 300 mV, which was insufficient to satisfy the required high rate discharge characteristic.
Further, as electrolytic manganese dioxide excellent in discharge characteristics, electrolytic manganese dioxide is reported wherein the (110)/(021) peak intensity ratio is larger than 0.50 and less than 0.70, and the (221)/(021) peak intensity ratio is less than 0.70 (Patent Document 1). The discharge characteristics of manganese dioxide disclosed therein showed some effects with respect to the discharge capacity under low loading of 30 mA, but they were no more than the characteristics which were not substantially different from the conventional products in constant current discharge of 500 mA.
Non-patent Document 1:Furukawa Denko Report, No. 43,p. 91-102 (May, 1967)Patent Document 1:JP-A-2007-141643Patent Document 2:U.S. Pat. No. 6,527,941Patent Document 3:JP-A-63-21224Patent Document 4:JP-A-2006-108083